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“There is no such thing as ideal body weight,” says James Rippe, a cardiologist and co-author of the Weight Watchers book “Weight Loss That Lasts.” “But your body does resist you when you are trying to lose weight. It gets used to a certain weight over a long period of time and then will defend that weight.”

In other words, the body’s “set point” can be lowered – or raised, he says, but it takes time to re-establish that new weight.

That period of time is at least six months, says Claire LeBrun, a registered dietitian and senior nutritionist with the Department of Surgery at the GW Medical Faculty Associates, whose patients include those who have undergone bariatric weight-loss surgery.

“I often ask patients, ‘What is the lowest sustained weight of your adult life?’ to get an idea of what is realistic” in terms of weight loss and maintenance, LeBrun says.

She refers to the body’s resistance to budging, weight-wise, as an “innate control mechanism that prevents us from starving and dying – part of our primitive biology.”

In the end, she says, some people might get down to their dream weight for a short period during the “action phase” of focused dieting but then can’t sustain it because the calorie restrictions are too severe once the body starts defending itself against weight loss.

“That’s why it’s important to set realistic goals,” she says. “The idea of the ‘ideal weight’ or ‘dream weight’ is really just an exercise in futility.”

But let’s say your goal is realistic and has been set by a nutritionist or other weight-loss professional and you are still plateauing. What could be going on?

Lise Gloede, a local registered dietitian, says, in that case, it’s time to revisit some of your nutrition and exercise habits and strategies.

“Maybe you are getting a little sloppy? Maybe you feel that you have been good for so long that you are letting some old habits back in?” she suggests.

If so, start – or go back to – a food log to keep track of even the small calorie intakes, including the 100-calorie snacks that can add up during the course of the day.

“Finishing the kids’ mac and cheese or caramel corn can make all the difference,” Gloede says.

Another reason that weight plateaus can be that the resting metabolic rate drops as the weight drops, says LeBrun.

In other words, you need fewer calories the less you weigh. So if you want to continue to drop weight, you have to drop calories and increase calorie-burn – the physical fitness portion of weight loss and maintenance.

“It’s also about body composition, not just weight,” Rippe says. “As you lose fat you want to maintain lean muscle mass through both strength training and aerobic conditioning.”

There are also age-related reasons for weight plateaus or even gains, Rippe says. “Hormonal changes as we age can make it more difficult to lose weight.”

Men, for example, don’t make as much testosterone as they age, which can lead to increases in body fat, especially in the midsection.

Rippe encourages people to clearly differentiate between weight loss goals for health reasons and those that revolve around vanity.

“They are both relevant issues, but they are different issues,” Rippe says. “One is where important health benefits are seen and the other is about vanity – wanting to look good in a bathing suit.”